I explained the distinction and why adding "the clade Theropoda" or "the group Theropoda" makes the singular okay.Â Higher category names MUST be Latin plurals, period, just as "data" is plural in correct grammar. Arguably, they could have written the sentence as "The Euarchonta hypothesis is similar to the morphology-based Archonta hypothesis"--but they didn't. "Theropoda are similar to the apomorphy-based Avepoda..." is correct grammar in this case.Â Add the word "clade" and the singular is required.

"'Within Euarchontoglires, our molecular results are the first to render robust support for the monophyly and internal structure of Euarchonta (3). Euarchonta is similar to the morphology-based Archonta hypothesis, but bats are
excluded." pg. 2348

[A point of grammar here: Archonta and Euarchonta are neuter plural substantives in Latin (not feminine singulars), so "is" should be "are"...]"

Really?Â They're discussing a clade, not the multiple individuals making up that clade.Â Would you say "Theropoda are similar to the apomorphy-based Avepoda, except Tawa and herrerasaurs are included"?Â We're not saying every one
of the multiple theropods is similar, we're saying the singular idea or concept is similar.Â Surely in these cases, however things work in Latin is superseded by English recognizing Euarchonta and Theropoda as single clades / hypotheses / suborders.